1. Field of the Invention
The invention relates to a transverse junction stripe laser (TJS) consisting of a semiconductor heterostructure comprising high bandgap cladding layers with an active low bandgap layer sandwiched in between, these layers being deposited on a surface of a semiconductor substrate. These lasers can be used in 1- or 2-dimensional arrays for providing high optical power output. The structure is particularly suited for GaAs technology devices.
2. Description of the Related Art
Optical light wave communications have become increasingly important in recent years for both telecommunications and data transfer within data processing systems. Light beams are generated, modulated to provide optical signals representative of data to be communicated, transmitted and detected before they are applied to electronic circuitry for processing. One of the important elements in these optical systems is a source for the light beam. Here, lasers of quite different structures, materials and concepts have been utilized with great success.
Increasingly important are p-n junction semiconductor lasers because of their compact size and because their technology is compatible with that of the associated electronic circuitry. In these devices, laser action is initiated by simply passing a forward current through the p-n junction, which itself represents a diode. As the threshold current is reached, stimulated emission occurs and a monochromatic and highly directional beam of light is emitted from the junction. These lasers can be modulated easily by modulating the current. Since photon lifetimes in these lasers are very short, modulation at high frequencies can be achieved. However, in homojunction lasers, comprising e.g. a GaAs p-n junction, the threshold current density has been found to be exceedingly large and to increase rapidly with increasing temperature. This imposes serious difficulties in operating the laser continuously at room temperature.
In order to reduce the threshold current density, multi-layer hetero-structures have been suggested and both single hetero-structures (e.g., n-GaAs/p-GaAs/p-AlGaAs) and double hetero-structures (e.g., n-AlGaAs/p-GaAs/p-AlGaAs) have been designed. These are fabricated by growing, on high-quality semiconductor substrates, successive epitaxial layers with different alloy compositions and dopings. Molecular beam expitaxy (MBE) processes have been used. Such hetero-structure lasers require only low threshold current densities because of (1) the carrier confinement provided by the energy barriers of higher bandgap material surrounding the active region, and (2) the optical confinement provided by the abrupt reduction of the refractive index outside the active region.
Most hetero-structure lasers are made in stripe geometries where the lasing area is confined to a narrow region under a stripe contact. The advantages of the stripe geometry include (1) a reduction of the cross-sectional area which, in turn, reduces the required operating current, and (2) an improved response time owing to small junction capacitances.
In these epitaxially grown hetero-structure lasers, the p-n junctions are normally formed at the horizontal boundaries between the active n-conductivity layer and a neighboring p-type layer, with the current flowing perpendicular thereto. However, more recently, so-called transverse junction strip (TJS) geometry lasers have been described which also consist of an epitaxially grown sequence of differently composed and/or doped semiconductor layers but with lateral carrier injection into and through the active layer in which a vertical p-n homojunction is provided. The main advantage provided by these TJS lasers over conventional vertical carrier-injection methods is that carrier and optical confinement are obtained not only in the vertical direction (through the AlGaAs-GaAs interface) but also in the lateral direction (through the P.sup.+ -p-n.sup.+ interface). This, in turn, leads to still lower threshold currents and further reduced capacitances.
In hitherto known TJS geometry lasers, the p-n junction in the active layer is formed by a Zn-diffusion which converts part of the layered n-type structure into p-type. The injection is then from the n-GaAs active layer to the Zn-diffused p-GaAs. Such TJS laser structures have, e.g., been described in an article "New Structures of GaAlAs Lateral-Injection Laser for Low-Threshold and Single-Mode Operation" by W. Susaki et al (IEEE Journal of Quantum Electronics, Vol. QE-13, No. 8, August, 1977, p. 587-591) and in a further article entitled "Transverse-Junction-Stripe-Geometry Double-Heterostructure Lasers with Very Low Threshold Current" by H. Namizaki et al (Journal of Applied Physics, Vol. 45, No. 6, June, 1974, p. 2785/86).
These TJS laser structures have been used successfully for low-threshold laser diodes with good aging properties. However, the fabrication process for the formation of the transverse p-n junction in the active layer is very complicated and difficult to control. In addition, since it uses a transverse p-n homojunction, no stable mode-guiding is provided in the transverse direction. Therefore, a complicated diffusion process with a stepped doping profile is required to build-in a small difference in the optical indices of refraction between the p- and n-regions. Since the index of refraction is not only material dependent but depends also on the injected carrier densities at the p-n interface, the laser diode can switch to oscillation in higher order transverse mode configurations at increased drive current. This causes severe non-linearities in the light-current characteristic and considerably increased optical noise levels.
These drawbacks are avoided by the laser structure of the present invention which is based on the observation that GaAs, doped with an amphoteric dopant such as silicon (Si), grown by MBE, exhibits a conductivity which depends on the crystallographic orientation of the GaAs substrate. For example, Si doping of a GaAs epitaxial film on a (111A)-surface can result in p-conduction and on a (100)-surface in n-conduction. This means that GaAs, grown on differently oriented GaAs crystal planes, can lead to the simultaneous deposition of p- and n-conduction material and to the formation of p-n junctions at the intersection of the regions of different doping.
This "crystal plane dependent doping" effect has been reported in an article "Lateral p-n Junction Formation in GaAs Molecular Beam Epitaxy by Crystal Plane Dependent Doping" by D. L. Miller (Applied Physics Letters, Vol. 47, No. 12, December, 1985, pp. 1309-1311) and in another article "Crystal Orientation Dependence of Silicon Doping in Molecular Beam Epitaxial AlGaAs/GaAs Heterostructures" by W.I. Wang et al (Applied Physics Letters, Vol. 47, No. 8, October, 1985, p. 826-828).
These references are representative of the present state of the art. They describe the effect of the crystal orientation on the doping but do not disclose or suggest the concept or structures of the present invention.
Also proposed herein is an application of the laser of the present invention in 1- and 2-dimensional arrays of phase-coupled lasers. Such arrays are capable of providing high power light radiation as needed for, e.g., optical recording techniques. The required power levels can, at present, not be provided by single diode lasers without the risk of potential mirror damage or lifetime degradation.
In order to obtain higher output power, linear 1-dimensional laser arrays have been suggested and analyzed, e.g., in an article "Experimental and Analytic Studies of Coupled Multiple Stripe Diode Lasers" by D. R. Scifres et al (IEEE Journal of Quantum Electronics, Vol. QE-15, No. 9, September 1979, pp. 917-922). Subjects of this study were coupled multiple stripe phase-locked, room-temperature lasers that provide well-collimated higher power beams.
In contrast to the known structures, the laser array proposed in the following can be extended to a 2-dimensional structure providing for a further increase in beam power.
It is a main object of the present invention to provide a semiconductor laser with high mode-stability for continuous operation at room temperature.
Another object is to provide a GaAs/AlGaAs transverse junction stripe (TJS) hetero-structure laser that can be produced using established, noncritical fabrication processes.
A further object of the invention is to provide a laser array capable of producing a highly collimated highpower beam.